The initial idea of King Peter I was to carve into the walls the names of all soldiers and officers who had perished in the Balkan Wars of 1912 and 1913. But, since the church was not fully completed, and since First World War followed (1914-1918), this idea had to be abandoned. The solution was to decorate the interior of the temple with mosaics, which would be a sort of a museum of reproductions of the prettiest frescoes of the Serbian medieval arts. Copies from 60 Serbian medieval churches and monasteries had been brought to the St. George church at Oplenac. The entire mosaic has 725 painted compositions (513 in the temple and 212 in the crypt), on which there are 1500 figures. The entire area of the mosaic is 3,500 square metres (38,000 sq ft); with 40 million various coloured pieces of glass which have 15 thousand different varieties of colour, making the most vivid artistic impression.

To the right side of the entrance, on the entire southern wall of the narthex, is the Painting of the Trustee, of King Peter I holding the model of his church on the palm of his left hand, wearing a crown and coronation ornaments. With his right hand, he is guided by St. George, to whom the temple is dedicated, and shown approaching the Mother of God, greeting him with Christ sitting on the throne.Tomb of King Peter I of Serbia.Tomb of Black George.

In the southern apse is the Gallery of the Serbian Medieval Rulers, an impressive line-up, with every one of them represented by their respective churches. The first on the left is Stefan Nemanja (Grand Duke, ruled from 1168 to 1196), wearing a priest garb of the Hilandar monastery, holding the Studenica monastery. Then, there is King Stefan the First Crowned (1196-1227) with the model of the Žiča monastery, then King Stefan Radoslav (1227–1234) with the narthex of Studenica monastery, followed by King Stefan Vladislav (1234–1243) with the Mileševa monastery, then King Stefan Uroš I (1243–1276) with the Sopoćani monastery; King Stefan Dragutin with the Arilje monastery (1276–1282), King Stefan Milutin (1282–1322) with the Gračanica monastery, King Stefan Dečanski (1322-1331) with the Visoki Dečani monastery, and two Tsars – Dušan The Mighty (1331-1355) with the Prizren church of St. Archangels; and Uroš The Weak (1355–1371), with the Matejić monastery near Kumanovo. The next depicted ruler, titled formally as Prince and informally as Tsar, was Lazar of Serbia(1371-1389) with the Ravanica monastery, his son Despot Stefan (1389–1427) with the Manasija monastery, followed by Despot Đurađ, with the church of Smederevo.

In the very calotte of the main dome is the Pantocrator (the Almighty Jesus Christ), copy of the thorax found in the Gračanica monastery. The face of Christ, 27 metres from the floor of the temple, looks very impressive and is of proportional dimensions. Although the diameter of this painting is 9 metres (29 ft) long, and although just the finger of Christ is 1.5 metres (5 ft) long, and the nose is 1.2 metres (4 ft) long, everything is harmonious. In the altar are the frescoes Lord's Supper and The way to the Golgotha. In the altar niche there is the 5-metre (16 ft) tall figure of the Divine Mother of God in a praying position (copy of the fresco from the Peć Patriarchate). Also, there is the Secret of the Holy Communion and the Communion of the Apostles with Bread and Wine.

These are just some of the compositions that make the rich interior of the church.

Besides the Mausoleum (St. George church), there are other objects that encompass the Foundation of King Peter I – King Peter's House, King’s villa, Queen’s villa, Vineyards, Vineyard Keeper’s House, etc. Visitors can also visit the historic town of Topola located nearby, a traditional stronghold of the Karađorđević family, ever since the time of Karađorđe, Leader of the First Serbian Uprising of 1804. There they can see what remains of the old Topola town, Karađorđe’s church and Karađorđe’s monument.